Team Fortress 2 is available on the Xbox 360 as part of Valve's Orange Box collection. I have no idea how large the playerbase on Xbox Live is but I don't imagine it's too big, the PC version is a lot more popular, especially after it went free to play. TF2 does have very modest requirements too, so even if you have a weak system, you might be better off grabbing it for free on PC rather than buying a copy of the Orange Box and paying for Xbox Live.